Gas Drainage Research Articles

Fluidity Influencing Factors Analysis and Ratio Optimization of New Sealing Materials Based on Response Surface Method

The borehole sealing material is one of the key factors affecting the gas drainage effect of a borehole. This paper takes the compressive strength, fluidity, expansion rate, and setting time of the sealing material as the main research indicators and explores the influence of each key influencing factor on the performance of the high-fluid sealing material through the single factor experiment method. Using the Design-Expert 8.0.5 Trial software designed orthogonal experiments and establishing a quadratic model between liquidity and each test factor, which showed the impact of each key factor on the fluidity. Finally, by adjusting the amount of admixtures, the optimal ratio of high-fluidity borehole sealing materials was obtained. The results showed that the key factors had the following order of significance: water – cement reducing agent > water – cement ratio > retarder > expansion agent . With the water-cement ratio and the amount of water reducing agent increase, the fluidity of the material will increase; and with the increase of the retarder and expansion agent, the fluidity will decrease. In actual use, the fluidity is the main factor, but the expansion rate, compressive strength, and setting time are also considered. The optimal percentages were found for the high-fluidity borehole sealing material: a water-cement ratio of 1, along with 0.03% retarder, 0.5% water reducer, and 8% expansion agent. These research results could provide a reference for improving the performance of gas drainage borehole sealing materials and enhancing the effect of gas drainage.

Characteristics of Seepage and Diffusion in Gas Drainage and Its Application for Enhancing the gas utilization rate

Although gas drainage technology has greatly developed, gas concentration and utilization rates are still very low, resulting in substantial quantities of low concentration gas emissions in coal mine. To study the roles of suction pressure on gas drainage, a mathematical model is developed in this study for coupled gas migration and coal deformation based on a dual-porosity medium. The simulation results reveal the initial gas production is mainly contributed by the seepage in the fracture, and then the dominating factor rapidly transitions to diffusion which provides relatively stable gas production in most drainage time. In addition, the increase in gas production is tiny while the gas concentration clearly decreases because of air leakage as the suction pressure increases. Therefore, a concentration-based suction pressure regulating method is proposed to extend the time period of effective gas drainage and increase the gas utilization rate through adjusting the suction pressure of the bedding borehole. Field tests were performed to constrain the gas drainage process under different suction pressures, and the results gained verify the effectiveness and applicability of this method. This study proves that the concentration-based suction pressure regulating method may be a promising technology to realize safe, economical and efficient underground gas drainage in coal mines in the future.

Research and application of gas drainage technology by downward drilling holes

Aiming at the undesirable phenomenon of low drainage concentration and poor effect in the downward boreholes in Panjiang mining area, the gas drainage technology of “full casing, two plugs and one injection, sealed hole under pressure” and pressurized air and water was proposed. And the on-site implementation was carried out in 21126 transportation jacking lanes of Tucheng mine. The research results show that the pressurized air blowing water technology effectively solves the problems of down-drilling slag and water accumulation, improves the gas drainage effect of the down-draining drilling, effectively solves the problem of regional outburst prevention, improves the efficiency of excavation, and promotes the safety of mine production.

A STUDY ON GAS DRAINAGE CONSIDERING COUPLING PROCESS OF FRACTURE-PORE MICROSTRUCTURE AND COAL DEFORMATION

Coal seam contains a large number of fractures, whose shape and structure of fissures are complicated, and they are the main channels for gas migration. Therefore, the quantitative analysis of the internal relationship between the microstructure and the macroscopic permeability is the key issue to increase the drainage volume. Some investigators discussed the permeability of coal seams based on the fractal theory, but the mechanisms of gas migration under the influence of fissure microstructures and coal deformation are still unclear. Moreover, most of the multi-process coupling analysis in the stage of Coal Bed Methane (CBM) mining was not taken into account. In this paper, the effects of fissure structures on the coal gas drainage rate in multi-field coupling are studied. Aiming at the mechanisms of gas drainage under the joint action of fracture structure, in-situ stress and adsorption deformation, we propose a two-scale multi-field coupling model, which takes into account the influences of micro fracture and pore structure. On this basis, we obtained the pressure evolution rule of coal seam pore system and fissure system and the distribution rule of coal seam displacement with drilling positions. Meanwhile, the effects of coal seam maximum fracture length, maximum pore diameter, the fractal dimension for fractures and fractal dimension for pores on coal seam extraction are discussed.

Differential Gas Drainage Technology for Upper Corner of Working Face by High Position Directional Long Borehole

In the mining process of high gas mining area, the gas emission from goaf is large, which leads to the gas overrun in the upper corner of the working face, and affects the safe mining of the mine. This paper studies the differential drainage of the upper corner gas by high-level directional long borehole, and then solves the problem of gas control in the upper corner. Taking a fully mechanized mining face as the research object, this paper analyzes the theoretical parameters of the overlying strata failure of the working face, and according to the production practice, the high position directional long drilling layer is arranged differently, and the influence of high position directional drilling on gas drainage effect in different layers is analyzed, and the best location in different distance and height area is obtained to guide the actual production work. The research results show that the differential drainage method of high position directional long borehole can effectively control the gas overrun in upper corner of fully mechanized mining face in thick coal seam.

Reasonable Arrangement of High-Level Orientation Extraction Boreholes of Pressure Relief Gas in Overlying Strata under High-Strength Fully Mechanized Mining in Low-Gas-Thick-Coal Seam

In order to solve the problem of pressure relief gas control under high-strength fully mechanized top-coal caving in low-gas-thick-coal seams, this paper studies the evolution of overburden structures and the distribution characteristics of fissure fields during the initial and stable period of working face by physical simulation and numerical analysis. The mathematical model of coupling between mining fracture field and pressure relief gas field is established. The results reveal the distribution characteristics of pressure relief gas field that considers mining-induced fissure field. According to the distribution of mining gas accumulation area, the high directional long boreholes have been put forward to control the pressure relief gas in goafs, and the effect has been tested. The results show that the initial pressure and three periodic pressures occurred from the cutting hole to 135 m in the initial mining period of the working face. The height of collapse zone developed to 22 m, and fracture height developed to 75 m. The development height of caving zone is stable at 25∼27 m, and the development height of fissure zone is stable at 75∼95 m. The process and distribution of pressure relief gas flow in goaf are obtained by solving the numerical model of pressure relief gas flow in mining fissure field. The gas accumulation area is located within 25∼55 m from return laneway and 25∼50 m from the roof of coal seam. After the implementation of high directional long drilling gas drainage technology in the initial mining period and the stable mining period, good results have been obtained in the gas control, where the average concentration of gas extraction is 5.8%, the average gas flow rate is 0.71 m3/min, and the gas concentration in upper corner and return air is less than 0.8%. The results can provide a reference for pressure relief gas control under similar conditions.

A FRACTAL MULTIPHASE TRANSPORT MODEL IN SHALE POROUS MEDIA WITH MULTIPLE TRANSPORT MECHANISMS AND ROCK–FLUID INTERACTION

Fluid transport in shales is complex due to the various storage spaces and multiple transport mechanisms, especially for multiphase transport during flowback and early stage of production. This study proposes a gas-water relative permeability fractal model during a gas displacing water process in shale gas reservoirs, with incorporations of (1) real gas transport controlled by Knudsen Number (Kn) and second-order slip boundary, (2) slip length for water phase transport, (3) a mobile water film with varying thickness due to rock–fluid interaction and (4) stress-dependence. Specially, the varying thickness of water film is determined according to the extended Derjaguin–Landau–Verwey–Overbeek (DLVO) theory through van der Waals, electrostatic and structural force during a drainage process. Moreover, the organic matter (OM) and inorganic matter (IOM) pore structures are considered with individual pore/tortuosity fractal dimensions. The proposed model is verified by comparing with an analytical model and experimental data. Results show that the decreasing pore pressure during depressurization brings a decline in gas relative permeability, while the decreasing pore pressure has little impact on water relative permeability. The impact of pore and tortuosity fractal dimensions of OM can be ignored compared with that of IOM. Furthermore, neglecting the mobile water film with varying thickness during a gas drainage process leads to an overestimation of gas relative permeability, especially at smaller pore sizes. This work presents a comprehensive model to determine gas-water relative permeability in shales by considering fluids/reservoir properties and rock–fluid interaction in full, which reveals multiphase transport mechanisms in the unconventional reservoirs.

Stress Distribution and Gas Concentration Evolution during Protective Coal Seam Mining

Coal and gas burst is one of the significant and catastrophic hazards in underground longwall operations. To date, the protective coal seam mining has been recognized as the most effective mining method for minimizing or even avoiding the effect of the coal and gas burst. In this paper, numerical modelling and field test were carried out for the longwall operation in Qidong Coal Mine in order to investigate the induced stress and coal seam gas drainage performance in the protected coal seam after the complete extraction of the protective coal seam. It was found that four stress zones can be classified in the protected coal seam being the original stress zone, stress concentration zone, stress relief zone, and recompaction zone. In addition, the monitoring data of gas concentration and volume change in the field agree well with the numerical modelling results.

A permeability evolution model of coal particle from the perspective of adsorption deformation

AbstractAs an important indicator, permeability can predict the gas drainage yield and prevent the mine gas disasters. We first reviewed our previous inversion method to investigate permeability coefficient of gas in coal particles; however, the relationship between permeability and adsorption pressure had not been summarized and explained theoretically. Here, a permeability evolution model including two crucial parameters of initial permeability and deformation coefficient was developed. The inversion gas permeability coefficients were converted into permeability, and then, the permeability of the same coal sample was fitted according to the evolution model. The results show that (i) the modeled results are matched reasonably well with the inversion permeability data, and thus, this model has been validated; and (ii) as volatile matter content increases, the initial permeability decreases exponentially, but the deformation coefficient basically grows in a linear trend. These two parameters are coupling to lead to a negative exponential decrease in permeability as gas pressure rises.

Seismic time-lapse monitoring of gas drainage in an underground coal working face

Gas drainage is a popular method used for preventing gas outbursts in coal seams. However, quantitative assessment of the drainage remains a challenging task. In this study, we conducted a seismic time-lapse monitoring study at the No. 15110 working face in the Sijiazhuang coal mine to evaluate the effectiveness of the gas drainage process. The result reveals that there is a remarkable difference in frequency bands and computed tomography (CT) image, inverted from the attenuation coefficients of the recorded channel waves, before and after the gas drainage. In the thick mylonitic coal zone, gas drainage operation generally decreases the wave frequency and wave attenuation. The comparable roof refractions from the monitored seismogram and CT-inverted velocity distributions demonstrate that changes in the coalbed roof during the gas drainage are minor. The attenuation differences are attributed mainly to the stress-relieving effects of the thick mylonitic coal zone. This observation is supported by the forward modeling of the channel waves. Time-lapse seismic monitoring employed in this study provides a useful tool for monitoring the impact of gas drainage on a coal working face, including its roof, for any coal mine with restively simple geological structures.

Numerical Simulation Study on the Distribution Law of Deviatoric Stress of Floor under the Influence of Mining

The Zhaogu No. 2 coal mine is a single thick coal seam mining mine with high gas content. Due to the lack of protective layer mining conditions, we can only arrange the floor gas extraction roadway (FGER) to extract the gas from the overlying coal seam of the FGER to ensure safe production. However, improper placement of FGER will cause water inrush from the floor of FGER. Given above contradictions, this paper analyzes the stress-strain relationship of the fractured rock mass in the caving zone and the stress-recovery characteristics of the goaf from the perspective of the structural characteristics of the overlying strata above goaf. Based on this, a FLAC3D numerical model of equivalent delayed filling of caving rock mass was established by using the double-yield model filling method. The distribution of floor stress under the influence of mining is obtained after the model calculation, i.e., with the increase of the distance from the floor, the ratio of bidirectional stress to the peak value, and the stability value decrease, but the decrease amplitude becomes smaller and smaller. Therefore, floor roadway used for gas extraction should be located 10-15 m below the floor. Combined with the distribution of floor strata in 11060 working face, it is finally determined that FGER should be located 13 m below the floor. Such arrangement of FGER can not only ensure the effect of gas drainage but also prevent the occurrence of water inrush from floor.

The effects of key rock layer fracturing on gas extraction during coal mining over a large height

AbstractTo prevent outbursts and extract the gas, it is necessary to investigate the structure of the key layer and the evolution of cracks during coal mining over a large height. A physical similarity model with a geometric similarity ratio of 1:100 and a strike length of 2 m was established on the basis of prevailing geological and mining conditions. Stress sensors and microseismic equipment were adopted to monitor the collapse characteristics of different key layers through variations in stress and energy. The research results showed that: The displacement of the overlying strata exhibited group motion characteristics marked by the key layer and there was zone of rapidly increasing stress in the goaf behind the working face before and after the fracture of the key layer. Through the analysis of microseismic events, fissure density, and fractal dimension, we found that: before and after the primary key layer fractured, the energy loss in the microseismic event accounted for 27% of the total, the fissure density also changed abruptly from 8 cracks/m to 10 cracks/m and the slope of the fractal dimension curves changed from 0.00243 to 9.94801 × 10−4. Then a gas drainage model suitable for this condition was constructed. When the primary key layer had not fractured, the cracks below fully developed to form a gas migration channel and a gas enrichment area, and gas boreholes could be arranged therein. After the primary key layer had been fractured, the rate of crack development decreased, the permeability would increase, and the gas concentration would increase. The boreholes could still be arranged in the gas migration channel. This study elucidated the control role of the primary key layer from multiple angles and provided experimental guidance for gas drainage over large‐mining heights and during complex key layer mining.

Evaluation of underground coal gas drainage performance: Mine site measurements and parametric sensitivity analysis

Underground gas extraction from coal formations has triple-effects involving mining safety, low-carbon gas capture and greenhouse gas control. Air leakage around the drainage borehole is a serious problem that continuously affects gas drainage performance. In this study, mine site measurement of gas drainage data is firstly performed in coal mine, and then a mechanism-based model is proposed to theoretically describe gas desorption and diffusion and flow through coal around the drainage borehole. Further, the propose model is numerically solved and verified with borehole drainage data measured in mine site. Followed this, the validated mechanism-based model is implemented to conduct parametric studies. The results showed that: (a) as the fracture permeability increases from 3 × 10−22 m2 to 3 × 10-14 m2, the air leakage flux increases from 7355 m3/d to 18,303 m3/d, and the gas concentration decreases from 46.9 % to 12.7 %, it indicates that changing the permeability around the borehole may be a wise strategy to control air leakage; (b) the coal matrix parameters (including permeability, sorption constant and radius of matrix) have a dynamic impact on gas drainage performance at the different stage of gas drainage. For example, the increment of methane production induced by increasing the sorption constants does not exceed 3.3 % at the drainage time ∼0.34 day; while the growth increases to more than 19.5 % at the drainage time ∼ 9.75 days; (c) at the initial stage of extraction gas production is mainly determined by the fracture flow. A higher permeability of coal fracture will incur more air leakage flux, reducing gas concentration in drainage borehole; (d) whereas, the matrix parameters dominate gas flow at a later stage. Increasing matrix permeability, sorption property or decreasing the matrix radius will enhance the gas exchange flux from the pore system of coal matrix to the fracture system, subsequently incurring a higher concentration/production of drainage gas. The simulated result and field tests demonstrates that sealing on the coal wall around the borehole can block a portion of air leakage paths and reduce air leakage linearly, which illuminates a more efficient strategy to minimize air leakage for underground gas extraction.

Experimental study on pore structure evolution of coal in macroscopic, mesoscopic, and microscopic scales during liquid nitrogen cyclic cold-shock fracturing

Liquid nitrogen cyclic cold shock is a new waterless fracturing method suitable for coal seams in arid areas. The pore structure of coal after cold shock plays a key role in gas drainage, but there is currently a lack of study combining macroscopic-mesoscopic-microscopic scale. Multiscale pore characterization will be helpful to clarify the damage mechanism of liquid nitrogen cold shock to coal. In this paper, the same coal samples were tested for ultrasonic inspection, stereomicroscopic imaging and nuclear magnetic resonance (NMR) after different cold shock times. The macroscopic damage were studied by ultrasonic wave; the changes of mesoscopic pores and fractures (0.1 ~ 10 mm) were studied by stereoscopic microimaging and digital image processing; the changes of microscopic pores (2 nm ~ 0.1 mm) were studied by NMR. Finally, the macroscopic - mesoscopic - microscopic pore characteristic parameters were coupled. The main results are as follows: With the increase of cold shock times, the P/S wave velocities decreased, the dynamic parameters decreased linearly, and the macroscopic damage of coal samples was obvious. The number of mesoscopic pores, surface porosity, and probability entropy increased. As can be seen from the T2 spectra of coal samples, the microscopic pores expanded and increased with cold shock times. The effective porosity increased, among which the increase of micropores contributed the most. The macroscopic wave velocity and dynamic parameters of coal samples were negatively correlated with porosities. The calculation models of T2 cutoff value were established by multiple linear regression analyses, which were in good agreement with the experimental values.

Discussion on Deviation Correction Technology of Long Borehole in Gas Drainage

To solve the drilling layer (especially small inclination to wear long borehole) deviation cause gas control drilling control range is not up to standard, large quantities and the problem of poor drainage effect in Xinji coal mine based on the analysis of borehole construction result in deviation in a variety of reasons, targeted in technology, management, system, equipment, and adopted a series of effective measures on the scene to “rectify”, puts borehole deflection can be controlled within the scope of reduced the drilling quantity, raise the drilling control ability, increase the gas control working time, realized the “technical economic integration”.

Study on the secondary plugging technology of drilling through the floor through drainage

According to the analysis and research results of the leakage channels of the boreholes in the coal mine face, the secondary plugging area of the leakage boreholes was determined, and the secondary plugging technology of “new extraction pipe + ‘one plug, one injection’” was formulated. And field test investigation was carried out. The research results show that after adopting the new secondary plugging technology for gas drainage, the concentration of gas drainage increases by 5~20 times, and the pure flow rate of gas drainage increases by 5~23 times, indicating that the drainage effect has improved significantly, which proves the feasibility of the secondary plugging technology.

Numerical analysis of coal seam heat injection

Coal seam heat injection is a new type of enhanced gas drainage method. In order to obtain the influence law of coal seam heat injection on the gas content in coal, the coupled equation of heat, fluid and solid multiphysics under coal seam heat injection is used to solve it with the help of numerical analysis software. the law of coal gas content change under the condition of coal seam heat injection is obtained: During the process of coal seam heat injection, the temperature of the coal near the heat injection borehole is relatively high. With the increase of the heat injection temperature, the adsorbed gas content in the coal gradually decreases, the free gas increases, and the residual gas content of the coal decreases more significantly. The extraction time was effectively shortened, and the expected goal of heat injection enhanced extraction was achieved.

Research on Reasonable Drilling Parameters of Hydraulic Reaming in Soft and Thick Coal Seam and Its Application in Field

In view of the fact that the outburst disaster of the soft 3# coal seam in the Sangshuping Coal Mine is mainly caused by in-situ stress, it is proposed to use hydraulic reaming, pressure relief and anti-reflection technology to remove some of the high geostress in the coal seam and improve gas drainage efficiency. Numerical simulation software was used to analyze the distribution of plastic failure and stress distribution of the coal body after reaming under the three-hole spacings of 4m, 6m, and 8m. The results show that the radius of the plastic failure zone around the borehole is about 2.2m when the rate of expanded coal chips is 3%; the width of the pressure relief area of a single reamed hole is about 3.2m. When the drilling hole spacing is 6m, it has a good pressure relief effect and reasonable amount of drilling work. After the reaming, the drainage concentration and flow rate are both improved. After reaming, the extraction concentration and flow rate have been improved. The hydraulic reaming technology can effectively solve the problem of outburst prevention in the thick seam of soft outburst dominated by ground stress in Sangshuping Coal Mine.

Study on gas characterization and fluorescence characteristics of intermediates in biogenic gas production from lignite by ultrasound assisted hydrogen peroxide pretreatment

In order to improve the utilization efficiency of coal by microorganisms and increase gas production, ultrasound assisted hydrogen peroxide oxidation of lignite was studied. The changes of accumulated gas, methane content, carbon dioxide content, and fluorescence characteristics of intermediate products in lignite and oxidized lignite system were studied by drainage gas gathering method, gas chromatography (GC) and fluorescence spectrophotometer. It was found that the cumulative gas could be divided into three stages: rapid gas production, slow gas production, and basic stable stage. The methane production rate of raw coal and lignite treated by ultrasound assisted hydrogen peroxide reached 987.23 μmol/g and 1290.22 μmol/g, respectively. The methane yield for oxidized lignite was 30.69% higher than that of raw lignite. The ultrasonic-assisted hydrogen peroxide pretreatment for lignite could effectively increase methane production. The analysis results of the fluorescence characteristics of the intermediates indicated that the total fluorescence intensity (FI) integral of intermediates decreased with the increase of time. The fluorescence peak positions mainly changed in tyrosine aromatic proteins and soluble microbial metabolites. Ultrasound-assisted hydrogen peroxide pretreatment lignite changed the amount of tyrosine-based aromatic proteins, hydrophobic organic acid fulvic acid, humic acids, and soluble microbial metabolites in the slow and stable stages of lignite biogas.

Effect of polyvinyl alcohol/aluminum microcapsule expansion agent on porosity and strength of cement-based drilling sealing material

In the process of gas drainage, the cement-based expansion sealing material with excellent permeability can effectively seal the cracks in the roadway pressure relief zone and the broken drilled area. However, expansion material has low strength and poor compactness. Therefore, the authors use microcapsule technology to modify the expansion agent to make the material expand after reaching sufficient strength, weaken the influence on the material structure and pores, achieve the coordinated development of expansion and performance, and improve the efficiency of gas drainage. In the paper, the uniaxial compression experiment and NMR experiment combined with the fractal dimension and material volume change is used to quantitatively analyze the influence of delayed expansion on the mechanical strength, porosity and pore connectivity of the sealing material. Experimental data proves the feasibility of applying microcapsule technology. The results show that the slope of the curve between the expansion rate and peak stress, porosity of ordinary expansion materials is 0.9931 and 0.0672, which is greater than 0.8925 and 0.0563 of microcapsule expansion materials. Microcapsule material has high strength, low porosity, and better sealing effect. Besides, the proportion of adsorption holes in the microcapsule material increases, and the seepage holes decrease.